Prismatic compensators are utilized in vertex refractometry to compensate for the prismatic effect of spectacle lenses. Such a compensation is necessary when measuring spectacle lenses having a prismatic effect. The compensation affords the advantage that it makes the measurement of astigmatic lenses easier.
Prismatic compensators include two wedge-shaped prisms (rotating prisms) which are utilized for determining the prismatic effect of the spectacle lenses to be measured and which are rotated through the same angle amounts in mutually opposite directions. The base position is determined by conjointly rotating the two rotating prisms.
In prismatic compensators of the state-of-the-art, the two rotating prisms are mounted in separate fittings. Each of these fittings is provided with a toothed rim at its outer periphery and both fittings are rotatable about a common axis. A spur gear or conical gear meshes with the two toothed rims and is rotatable by means of a rotatable knob. In this way, the toothed rims and therefore the rotating prisms are rotated in mutually opposite directions by the same angular amounts.
However, in spite of very precise manufacture of the known toothed rims, pitch errors occur during the assembly of the prismatic compensator when one of the toothed rims is brought into engagement with the other toothed rim via the conical gear or spur gear. Such a pitch error can only be corrected by a complex alignment of the toothed rims with respect to each other.
Furthermore, the play between the conical gear or the spur gear and the toothed rims can have a disruptive and falsifying angle variation even when a pitch error is corrected.
The spur gear and the toothed rims are expensive because of the required precision and require a great deal of space as well as being heavy when compared to the remaining components.